Problem: The scale on a map is 8cm : 2km. If two cities are 80cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 8cm on the map corresponds to an actual distance of 2km. A distance of 80cm on the map is the same as 10 $\cdot$ 8cm. The actual distance between the two cities is 10 $\cdot$ 2km, or 20km.